The present invention relates to a method for generating steam with concentrated solar radiation in which a stream of air is guided through an absorber of an air heating device heated by concentrated solar radiation and heated by passing through the absorber, the heated airstream is then cooled by generating steam and at least a portion of the cooled air stream is returned to the upstream side of the absorber.
In the book "Second Generation Central Receiver Technologies" (Becker, Klimas, published by Deutsche Forschungsanstalt fur Luft-und Raumfahrt, Verlag C. F. Muller GmbH, Karlsruhe 1993, Pages 1-2, FIGS. 1-2) such a method is disclosed. When using a volumetric air heating device the heated air is guided with blowers directly to the steam generator and/or to a thermocline storage device. The storage device serves as a compensation means for differences between the energy supply and usage over time and is switched in parallel to the steam generator.
In certain devices according to FIGS. 4-7 on pages 4-41 of the aforementioned book a burner operated with fossil fuels is arranged upstream of the steam generator. With such a burner the production of electricity in cooperation with the thermocline storage device can be maintained when no solar energy is present or no heat energy can be removed from the storage device. In the known devices the receiver, with which the solar radiation collected with a solar panel field is concentrated, is arranged on top of a high tower and the steam generator is positioned at the bottom of the tower.
The steam generator used for producing steam requires with respect to its service life and with respect to protection of the steam turbine loaded therewith a substantially constant air inlet temperature and a substantially constant air mass flow at least within short time intervals. This can not be insured with an air heating device because it can only be operated either with a constant mass flow or a with a constant air outlet temperature. For a constant mass flow the air outlet temperature changes with the changing solar radiation received by the solar panel field. Temperature fluctuations of the heated air may occur when clouds suddenly cover the large surface area of the solar panel field because the temperature of the air leaving the air heating device then drops. This results in a change of the average temperature with which the steam generator is loaded as well as in a change of the three dimensional temperature profile, especially when only a portion of the solar panel field is shaded by clouds. It has been shown that the conventionally used absorber for the air heating device in the form of woven metal etc. have a to low storage effect so that a change of the radiation energy very quickly effects the air outlet temperature. For a controlled and thus constant air outlet temperature the mass flow is the variable parameter.
It is thus the object of the present invention to provide a method of the aforementioned kind with which a quasi stationary operation of the steam generators/turbine is possible, i.e., an operation with such temperature transients that are acceptable to the steam generator/steam turbine.